Spectral luminous efficiency of human eyes varies in photopic, scotopic, and mesopic visions. In photopic vision, cone cells are in function to sense colors. In scotopic vision, although colors are not perceivable because no cone cells are in function, rod cells operate to improve color-sensitivity. In mesopic vision, which is an intermediate state of the photopic vision and the scotopic vision, both the cone and rod cells are in function. Herein, the spectral luminous efficiency in photopic vision has a peak wavelength of about 555 nm. On the other hand, the spectral luminous efficiency in scotopic vision has a shifted peak wavelength of about 507 nm. Such phenomenon is well known as “Purkinje phenomenon.”
As an index of the brightness sensed by human eyes, luminance and illuminance are typically employed. As for conventional luminance and illumination meters, however, the luminance and illuminance are measured based on the spectral luminous efficiency in photopic vision. Accordingly, measurement values of the luminance and illumination meters in scotopic and mesopic visions differ from the brightness actually sensed by human eyes.
In a conventional example disclosed in Japanese Patent Application Laid-open No. H2-205731, mesopic-vision equivalent luminance of a light source is calculated from measurement values of luminance and correlated color temperature, based on such knowledge that the correlated color temperature and the luminance of the light source (luminance in photopic vision, i.e., photopic-vision equivalent luminance) correlate with the mesopic-vision equivalent luminance.
However, since a relationship between the spectral luminous efficiency in mesopic vision and the color temperature of the light source is indefinite, such conventional example is not widely spread due to its low validity of the measurement value.